For at least the past twenty-five years, there has been in the U.S. and around the world a rapidly growing electronics industry in which an integral and important part has been the development and manufacture of printed circuit boards. These printed circuit boards are typically formed of electrically conductive traces supported by nonconductive substrates (i.e. boards) in such a manner that electrical and electronic components, for example resistors, capacitors, integrated circuits, and transistors, may be mounted so that the result is an electrical circuit incorporating these components. The electrically conductive traces are the "wires" of the circuit, and the board provides structural integrity, facility for mounting to chassis frames, and support for interconnection to other circuitry. Printed circuit boards are an integral part of almost every electronic product and are often the most complicated and expensive parts of the entire device. All of the working parts and electronic memory of many computers, for example, are implemented on one or more printed circuit boards.
The desktop computer is a good example of a product that makes extensive use of printed circuit boards. The development of integrated circuits has been instrumental in the development of desktop computers, which are based on powerful central processing units (CPUs) and memory chips capable of storing large amounts of digital information. The motivation for more and more processing power, such as sophisticated graphics generation and display, has led to miniaturization techniques to put more and more devices on a single integrated circuit chip. By the same motivation, larger numbers of memory chips and processors have been brought into action in limited space environments, such as the chassis of a desktop computer, by innovative techniques for increasing the density of printed circuits.
Just as in the development of any product, certain techniques have emerged over the years for producing printed circuit boards that have proven to be useful and effective, and have come to be used by many manufacturers. According to one such process, as described in the Handbook of Printed Circuit Manufacturing, by Raymond N. Clark, published in 1985 by Van Nostrand, a typical printed circuit board begins with a sheet of nonconductive polymer material, such as fiberglass reinforced epoxy. The material that will eventually form the traces is copper foil, and a thin sheet of the foil is mounted to one or both sides of the fiberglass sheet, typically with epoxy resin as an adhesive to form a blank. Holes are usually drilled into the laminated blank. Some holes are for such purposes as registration, alignment and mounting of the board to other elements of an assembly, many are for accepting wire leads of electrical components to be finally mounted to the finished board, and many are to provide for electrical contact through the laminate between the eventual traces on one side and the traces on the other.
After holes are drilled, electroless plating operations are used in those cases where needed to plate conductive metal, usually copper, through the holes. Imaging techniques are subsequently used to lay a pattern over the copper foil defining the circuit traces to be formed. The two commonly used techniques are silk screening and dry film photoresist. In each of these, the pattern, called artwork, has to be separately determined and rendered in masks as part of the process. In the silkscreening process, plating resist material is applied to the foil through openings in the mask. In the dry film photoresist process the entire copper foil area is coated, then a pattern is cured through a mask by radiation, such as ultraviolet light. The uncured resist is then washed away. In either case the result is a pattern of plating resist material on the foil, covering those areas of the foil that will be eventually be removed to leave the circuit pattern. The resist does not conform the pattern of the circuit traces, but rather to the pattern that is not the circuit, i.e. the negative of the traces.
The next step in the typical process is the plate up of the circuit pattern. Conventional plating techniques are employed to increase the thickness of the exposed copper foil in the circuit pattern to a predetermined thickness to provide adequate current carrying cross-section and structural integrity. The plating operation is typically finished by applying a layer of a material such as tin-lead alloy (solder) over the traces. The resist is then removed, usually by solvent wash, and the foil under the resist is removed by chemical etching. The overplate of solder material protects the circuit traces from the etchant. The conductive circuitry is left on the surface or surfaces of the board.
There are a number of operations performed that are not detailed in this description, such as reflow and solder mask. The purpose here is not to teach the manufacture of printed circuit boards by conventional techniques, but to show that it is indeed a complicated, time consuming, and expensive process. A single board, even mass produced, can cost hundreds of dollars, and to apply these techniques to a limited quantity of boards can costs thousands per board.
In the development of circuit board production techniques, particularly to increase complexity and density, a number of innovations have been made. One development path involves the lamination of boards together so that there are inner layers of complex circuitry. Connection to board components and to other circuitry in other layers is made through holes and vias through the nonconductive board materials. These are known in the art as multilayer boards, and through their use, the amount of circuitry that can be accommodated in the same board space has been increased several-fold.
Another development has been the use of polymer thick film materials in the preparation of printed circuit boards, particularly multilayer boards. Polymer thick films are polymer materials (i.e. plastic resins) that are thixotropic through the use of additives such as when loaded with conductive materials, e.g. fine particles of silver, or with thixotropic additives (e.g. fused silica). These are silk screened directly onto the surface of a nonconductive board, forming the conductive traces directly. Used in multilayer technology, layers of silk screened circuits may be interlayered with other materials and printed circuits in multiple layers, and interconnection between layers is made by vias and drilled and plated holes. In general, polymer thick film multilayer boards can cost half of what laminated copper multilayers cost.
With these new developments and others, there are, however, still significant problems that haven't been adequately addressed, problems which have carried over from one technology to plague the next. One of these is in the application of artwork to the formation of the traces on a board. The first steps to the production of a board are engineering steps. The theoretical circuit is conceived and circuit performance is calculated. Typically, then, components are specified from commercially available stocks, such as one manufacturer's CPU, another's DRAMs, and other elements from still other manufacturers, and calculations are made to determine the desirable traces. There may be, for example, certain restrictions on the length of certain traces where ultra high speed is required in the transmission of signals, or in the cross-section and conductivity, where high current loads are to be borne. There are other engineering considerations, too, such as expansion and contraction of traces versus the supporting materials, and the requirements for cooling and heat dissipation. In these calculations, a design engineer is limited, too, by the nature of the prototyping and production equipment that is to be used to implement the design. These limitations are known as design rules.
In present work environment, the design engineer is now aided in many cases by new and powerful computer programs that take all the design rules into account, calculate, and prepare graphic displays of trace patterns. The patterns created are called routes and the software tools are called routers. A router engine is the basic calculator tool for quickly doing the multitude of alternative layouts that are possible, still obeying all the router rules. Of importance is the fact that at the end of the routing and the iterative process of choosing the options that a computer creates, the route has to be implemented in artwork to be transferred to the board prototype or production process. The conventional process requires masks for the photoresist or for the silkscreening of plating resist. The polymer thick film process requires silkscreens for the application of the uncured materials to the board surfaces.
The implementation of the artwork introduces expensive and time consuming intermediate steps that also increase chances for error, such as misalignment.
Another problem that still prevails, and which becomes ever more complex, is the absolute necessity of verifying a design before dedicating the design to a hugely expensive and cumbersome production process. This process is known as breadboarding, which is the practice of preparing a limited number of boards in, hopefully, a relatively inexpensive way, to verify that a circuit which appears to be functional and for which engineering calculations have shown it to be functional, actually works in the real world. One way that has been used to breadboard is to mount the circuit components on standard boards without circuit traces, but with edge pads and some other more or less standard features, and to connect the components by hand soldering or wire wrapping, using fine wires and other hand manipulatable trace materials. This process is, of course, cumbersome and prone to error. Other breadboarding techniques involve essentially the same techniques that are used in production, but incorporating manually operated systems and smaller-than-production dedicated facilities, such as small plating tanks to do one or a few boards at a time. Again the process is expensive and time consuming.
Another problem that is very typical occurs when breadboarding is finished, the iterative correction process is complete, and a large number of boards have been prepared. An engineering change is often required, such as adding a component or two to upgrade performance. The result is a large stock of finished boards, representing a large amount of money that must either be scrapped, at an unacceptable cost, or reworked. This rework process to make the stock into usable or salable product involves cutting traces and adding jumpers. The process of doing the cuts and jumpers is prone to error, because it is most generally a hand work process, and is quite expensive for the same reason. Moreover, the changes made, since they are most often made in a way foreign to the production process, are clearly visible. The presence of cuts and jumpers is considered by many to be a good way to judge the engineering foresight of a company that manufactures printed circuit boards. Many product reviewers also judge products partially according to the prevalence of cuts and jumpers on the printed circuit boards used.
What is needed is an apparatus that can be computer controlled so that computer aided engineering and computer generated routing and associated artwork can be generated directly on the control computer, or loaded to the control computer, so that traces of a printed circuit board can be generated directly by the apparatus. Such an apparatus would eliminate the intermediate steps of implementing the artwork in other forms, such as masks for applying photoresist or plating resist, and the resist and plate-up operations as well would be eliminated. To achieve maximum utility, such an apparatus should also generate traces that can cross one another (i.e. provide crossovers), then the density of multilayer boards could be provided as well. The direct writing of traces from digitized data would integrate all of the complicated steps of preparation of printed circuit boards, and would particularly facilitate the process of breadboarding. Such a direct circuit writing apparatus could also significantly improve the process of providing rework in the form of cuts and jumpers. On a board made by such a circuit writer, cuts and jumpers could be programmed and automated, and would be difficult to distinguish from original traces. Such an apparatus could also be used on conventional boards (with accessible traces) to provide jumpers, if appropriately programmed and automated, thereby eliminating much expensive and error-prone hand rework.